1. Field of the Invention
The present invention relates to a data processing apparatus and, more particularly, to a data processing apparatus for processing a plurality of kinds of data of different processing speeds such as image and audio data.
2. Related Background Art
In a television conference or a television telephone, an image, particularly, a moving image and an audio signal are simultaneously transmitted. In particular, the moving image is compression coded in order to reduce a transmission data amount.
As a compression coding method, a method in which a discrete cosine transformation, a quantization, and a variable length coding in a narrow sense such as a Huffman coding or the like are combined is widely known.
FIG. 1 is a block diagram of a conventional data transmitting apparatus for time division multiplexing an image and an audio signal and transmitting.
A video signal is input to an input terminal 10 and is converted into a digital signal by an A/D converter 12. The digital signal is compression coded by a signal processing circuit 14.
A code amount of data which is generated by the signal processing circuit 14 fluctuates in accordance with a picture pattern of the image of the video signal which is input to the input terminal 10.
The code amount of data which is output from the signal processing circuit 14 is adjusted in accordance with a data accumulation amount (data occupation ratio) in a buffer memory 16.
That is, a latency data amount in the buffer memory 16 is fed back to the signal processing circuit 14 through a feedback path 18.
The signal processing circuit 14 adjusts the amount of codes which are generated in a manner such that the latency data amount in the buffer memory 16 lies within a predetermined range, in other words, an overflow of the storage data doesn't occur in the buffer memory 16.
On the other hand, an analog audio signal is supplied from an input terminal 20 to an A/D converter 22 and is converted into a digital signal.
A signal processing circuit 24 executes a predetermined process for transmission, for example, a pulse code modulation (PCM) to the audio data which is output from the A/D converter 22.
A delay circuit 26 delays an output of the signal processing circuit 24 by a predetermined time and supplies the delayed signal to a (b) contact of a switch 28. An output of the buffer memory 16 is supplied to an (a) contact of the switch 28.
The information data which is time division multiplexed by the switch 28 is output to a transmission path from an output terminal 30.
The operation in the buffer memory 16 will be briefly explained with reference to FIG. 2.
A capacity of the buffer memory is set to Bc, a data rate on the writing side is set to Dw, a data rate on the reading side is set to Dr, and a target data occupation ratio is set to 1/2 in consideration of a response speed of the feedback path 18.
Since the data rate Dr on the reading side corresponds to a transmission rate, it is generally constant. The data rate Dw on the writing side fluctuates around Dr as a center due to the feedback control by the feedback path 18. A data latency delay time of the buffer memory 16 is equal to Bc/Dw (or Bc/Dr).
In case of executing a data amount control as mentioned above, since the image data passes once through the buffer memory, a latency delay time occurs and a time deviation occurs between the image data and the audio data.
For example, when there is a possibility such that the transmission rate changes as in the case of using transmission media of different transmission rates or a transmission medium such that a plurality of transmission rates can be selected, a fluctuation range of the time deviation between the audio data and the image data increases.